A voltage phase control method is known as one of control methods for a motor and the like. In the voltage phase control method, a phase command value is calculated based on a torque command value and an amplitude command value is calculated by use of a predetermined modulation rate. A voltage corresponding to the phase command value and the amplitude command value is applied to the motor. Generally, in such a voltage phase control method, in order to improve accuracy of a rotation control on the motor, a feedback control is performed on the phase command value. More specifically, a torque estimated value is found from a current flowing into the motor, and the phase command value is controlled so that a deviation between the torque command value and the torque estimated value is reduced. (JP 2013-137129 A)